Problem: Solve for $x$ and $y$ using substitution. ${-4x-y = 0}$ ${y = 3x+7}$
Since $y$ has already been solved for, substitute $3x+7$ for $y$ in the first equation. ${-4x - }{(3x+7)}{= 0}$ Simplify and solve for $x$ $-4x-3x - 7 = 0$ $-7x-7 = 0$ $-7x-7{+7} = 0{+7}$ $-7x = 7$ $\dfrac{-7x}{{-7}} = \dfrac{7}{{-7}}$ ${x = -1}$ Now that you know ${x = -1}$ , plug it back into $\thinspace {y = 3x+7}\thinspace$ to find $y$ ${y = 3}{(-1)}{ + 7}$ $y = -3 + 7$ $y = 4$ You can also plug ${x = -1}$ into $\thinspace {-4x-y = 0}\thinspace$ and get the same answer for $y$ : ${-4}{(-1)}{ - y = 0}$ ${y = 4}$